This invention relates generally to the field of fabrication of thermoplastic resins, and more particularly to the extrusion of foamed fluorocarbon resins commonly used for providing an insulative covering over conductive wires and cables.
The use of foamed fluorocarbons in the field of electrical insulation is well known, and techniques for extruding such materials at relatively high speeds are widely used in the art. In the field of wire coating, the use of foamed materials has been found particularly advantageous because of the favorable dielectric qualities obtained. This is normally accomplished by passing the conductor or wire to be covered through the extrusion die, with the extrudate from the die flowing about the wire to be formed in situ.
For reasons of economic productivity, it is desirable to extrude in such manner at as high a rate as possible. As disclosed in U.S. Pat. No. 2,991,508, granted July 11, 1961 to R.T. Fields, et al., it is essential that the extrudate have a smooth surface to be commercially acceptable. For most polymers as the rate of extrusion is increased, the outer surface becomes progressively rougher. The point of maximum rate resulting in acceptable smoothness is referred to as the "critical extrusion rate". This rate can be increased by increasing the melt temperature. However, at a certain point the melt viscosity of the extrudate becomes so low that the extruded article will not retain its shape, or the resin itself becomes degraded.
A material commonly used is FEP (Fluoroethylene Polymer), which is a mixture of tetrafluoroethylene polymer and tetrafluoropropylene. As described in the above mentioned patent, it is known to extrude at a rate far above the "critical extrusion rate", at a level known as the "super extrusion shear rate", as well as at reasonable levels above that rate.
Although the super extrusion shear rate is a function of temperature, the "super extrusion flow rate" is not. Rather, it is a function of the temperature of the extrusion die. It is therefore necessary to provide means for controlling the temperature of the extrusion die in order to control the super extrusion shear rate.
The above described process has not been without complications. In order to accomplish sufficient melting, mixing, gas injection, pumping, and mechanically induced shear within the extruder, and to process the FEP foam, the barrel and screw used to flow the extrudate have required considerable modification.
It has been determined that the use of traditional design criteria for the screw extruder has limited the capacity of the extruder both to mechanically flux and pump the compound. In addition, with the use of lower viscosity in the extrudate, there arises a tendency of the material to "backflow" or slip between the surfaces of the screw flights and the inner surface of the barrel of the extruder, with resultant difficulty in maintaining the necessary high delivery pressure on a relatively low viscosity extrudate.
Another problem has emerged in the difficulty in maintaining uniformity in the mass of the melted material. In the case of extrusion about a wire core, the flow of material must change direction through 90.degree. in its path of travel, which tends to affect the homogeneity of the mass of the extrudate.
Still another problem has been the increase in criticality of die temperatures at various points of extrusion and at the point of mixing.